Fuel efficiency is a significant element of the overall economics of a road transportation vehicle, e.g., semi-truck (“tractor”) with a trailer. Fuel efficiency of the road vehicles is mostly determined by engine efficiency, rolling resistance of the wheels, stop-and-go traffic (i.e., acceleration and deceleration), drag coefficient, and trailer loading. At relatively high and steady velocity, typically referred to as “highway driving,” the air flow drag often dominates energy losses of the vehicle.
Many truck manufacturers measure the drag coefficient and pressure coefficients to characterize and possibly improve the aerodynamics of their vehicles. FIG. 1 illustrates conventional technology for measuring a drag coefficient for a combination of a tractor 10 and a trailer 11 travelling in a direction 40. With the illustrated conventional technology, either the trailer 11 or the tractor 10 is equipped with two probes: a total pressure probe 20 that faces the direction of the travel and a static pressure probe 21 in a close proximity. The pressure probes 20, 21 can be elevated above the tractor/trailer combination into airflow that is less influenced by the presence of the tractor/trailer combination. The probes 20, 21 are typically combined in a Pitot-static probe, where probe 20 measures total (stagnation) pressure of the airflow, i.e., the pressure of the air particle having decelerated to zero velocity at the tip of the probe, and probe 21 measures the local static pressure. The local dynamic pressure (PDYN) is then determined by the difference between the total pressure (PTOT) sensed at the tip of the probe 20, and the local static pressure (PST) obtained from probe 21, which can be expressed as:PDYN=PTOT−PST  Eq. (1)
A drag coefficient (CD) of the tractor/trailer combination can be calculated based on knowing PDYN (from PST, PTOT), the aerodynamic force, and relevant dimensions of the tractor/trailer combination. However, the accuracy of these calculations depends, among other factors, on the accuracy of the PST (static pressure) measurement. For accurate calculations, the PST should correspond to the static pressure of the freestream airflow that is sufficiently far away from the moving object, therefore not being disturbed by the moving truck/trailer combination. With conventional technologies, the static pressure probe 21 is placed at a location on the vehicle that is presumed to have a relatively stable static pressure. However, the probe 21 is still exposed to the fluctuating and non-representative free flow pressure due to, for example, wind conditions and unsteadiness of the flow field around the vehicle. Stated differently, readings of the conventional static pressure probe 21 are still within the “disturbed” airflow around the truck/trailer combination, thus generally not providing an accurate measurement of the freestream static pressure PST∞. Additionally, the selection of a location for the static pressure probe 21 necessarily introduces a biasing error, because the reading of the probe is generally influenced by its location near the truck/trailer combination while in motion.
The errors in determining the PST∞ (freestream static pressure) can be eliminated by placing the static pressure probe 21 sufficiently away from the trailer/tractor combination, into the freestream. However, such a placement results in very long and impractical pressure probes 21 making the measurements expensive and cumbersome. Accordingly, there remains a need for accurate and cost-effective determination of the drag coefficient (CD) and pressure coefficients (CP) of the truck/trailer combination.